The term filtration is often considered the primary function of a geotextile used in subsurface drainage applications and for erosion control beneath hard armor along inland waterways and in coastal erosion control systems. In all these cases, geotextiles are used to retain soil particles in-place and prevent their movement caused by the erosive force of seepage exiting from the subgrade, or the traction of overland flow against particles on a ground surface, or the impact of wave action on a ground surface. Prior to geotextiles, graded aggregate filters were the conventional engineering design solution to soil retention. But despite being the conventional solution, the frequency of use for a properly designed and installed graded aggregate filter was very limited due to availability, and costs of materials and/or installation.
The real functions of the filter media are retention and seepage, i.e., to hold the particles in place while seepage exits a soil mass. The required gradation of a graded aggregate filter to provide these functions is a fine pore structure adequate to retain soil particles in place while allowing the free seepage of water as it exits the soil mass without a buildup of hydrostatic pressure at the soil-granular filter interface. If seepage is blocked, hydrostatic pressure will build between the soil particles, reduce their interface friction, and weaken the stability of the soil mass (e.g., subgrade, foundation, or earth slope). The result will be an unstable soil mass and possible failure under the loads being supported by that soil mass.
A geotextile filter must meet the same retention and seepage functions as a graded aggregate filter. When the proper geotextile is specified and installed, it easily provides in-place particle retention to the adjacent soil particles, and the geotextile pores allow seepage to exit freely from the soil mass. This retention of soil particles and seepage of moisture from the soil mass allows for the unrestricted release of hydrostatic pressure from within the soil structure. Both the availability and simplicity of installation for a geotextile make it the preferred filter medium for filtration in subsurface drainage and erosion control applications when compared to the cost of a properly designed and installed conventional graded aggregate filters.
In some cases, voids or openings in the soil mass develop between the soil surface and the filter medium, especially when geotextiles are used. Under these circumstances, subsurface erosion can develop at the soil surface that is not restrained by the filter. If the soil particles are coarser than the geotextile pore structure then the geotextile will actually filter or retain the eroded particles from the fluid as it tries to permeate through the geotextile. These voids in the soil-fabric interface are typically a result of poor soil surface preparation or inadequate compaction before or immediately following placement of the geotextile adjacent to it. These circumstances require remedial actions to prevent long term subgrade or foundation instability problems.
Geotextiles used for subsurface drainage and erosion control are typically nonwoven and/or woven monofilament fabrics with a pore size slightly smaller than the average particle size within the soil mass being retained, and geotextile permeability is significantly greater than that of the soil being protected.
Sedimentation control applications use a geotextile to provide the functions of retention and filtration of eroded particles from a sediment laden fluid as it runs off a freshly graded construction site. The runoff is traveling under gravity flow conditions until the fabric filter mounted on its vertical structure (e.g., silt fence) retains the slurry of soil and water. Once retained, the slurry enters a steady state condition and the solid particles either settle out of suspension to the ground upstream of the vertical structure, or are filtered from the retained slurry as its carrier fluid seeps through the pores of the filter medium. Despite the reference to retention and filtration by the geotextile for sediment control, the fabric functions and the geotextile properties required are significantly different from those for subsurface drainage and erosion control beneath hard armor.
In sedimentation control the geotextile is used as a permeable barrier to span the flow path of a sediment laden fluid. A vertical structure or vertical support elements support the geotextile upright while the fabric provides retention of the sediment laden fluid, allowing the sediments to settle from suspension. Soil particles that remain in suspension upstream of the geotextile are filtered from the retained sediment laden fluid as its water seeps slowly through the fabric pore structure. These conventional structures are referred to herein as linear retention and filtration systems and include silt fences, curb inlet sediment filters, and temporary check dams.
The most visible to the public eye of linear retention and filtration systems is the silt fence, seen around most all earthwork construction sites in the U.S. Their prominence is a result of EPA regulations enacted in 1972 mandating the use of sediment control measures to contain sediment runoff and prevent contamination to adjacent properties, streams, and waterways. In 1975 the EPA approved the use of a geotextile lined fence to meet these regulations, and since then the use and prominence of geotextile silt fences have become one of the standards of practice in earth work construction.
These original EPA regulations have been adopted by other governing bodies on federal, state and local levels, and today each has its own version of those same regulations. Most of the regulatory requirements include detailed specifications for the sediment control structure, the geotextile used for retention and filtration in the system, the structural components of the system, and specific details regarding installation of each of the systems components.
The silt fence is a linear system that is basically a two dimensional structure of specified length and height with the geotextile installed in a vertical posture above ground to provide a retention and filtration barrier for sediment runoff. The silt fence is held vertical by metal or wooden fence posts driven firmly into the ground at nominal spacings (e.g.: 4 to 10 feet). A wire mesh is often secured to the posts before the geotextile to hold the fabric vertical and give it added strength and stability to support the loads from the sediment laden fluid it retains. The bottom of the fabric is buried into the ground (i.e., toe-in) with the backfill firmly compacted into the fabric toe-in trench to prevent erosion and washout of the toe-in and subsequent erosion beneath the system. The geotextile on the structure retains sediment laden runoff resulting from rainfall events. The suspended sediments in the retained runoff either settle to the ground or are filtered out as carrier fluid seeps very slowly through the geotextile filter media. The filtration process described above prevents sediment contamination downstream.
Constant seepage through the geotextile is necessary for filtration of the sediment laden fluid that the system retains. When the retained flow carries sediments in suspension, the geotextile will begin the retention and filtration process, and a filter cake of particles from the sediment laden fluid forms on the fabric upstream surface. The porosity and permeability of the filter cake decrease with time and the eventual result is a retention system with very low to no measurable seepage passing through it. This creates an above ground sediment pond. Inadequate seepage rate through the filter cake can lead to excess buildup of retained sediment laden fluid followed by system failures due to overflow or collapse caused by the lateral forces from the retained fluid upstream of the retention structure (e.g., silt fence).
Proper attention must be given to burial and compaction of backfill at the toe-in of the filter medium at the base of above ground retention and filtration system. Weakened condition of toe-in backfill atop the geotextile at the base of a retention and filtration system due to inadequate compaction or saturation of the soil backfill can lead to further reduced backfill density due to prolonged ponding of retained sediment laden fluid upstream of the silt fence.
These toe-in problems will result in localized system failures due to scouring and erosive channels under-cutting the fabric at its toe-in at the base of the retention and filtration system. These types of failure can progress to more complete system failures if not corrected in time.